js动态获取文本框的值 js获取月份的天数...

<script type="text/javascript" language="javascript">
    function txtChange()
    {
        var time=document.getElementById("txtWorkTime").value;
        var days=document.getElementById("txtWorkDays").value;
        var rate=document.getElementById("txtRate").value;
        var extraTime=document.getElementById("txtExtraTime").value;
        if(time=="" ||days==""||rate=="")
        {
            if(extraTime=="")
            {
                 document.getElementById("txtTotalTime").value="0";
            }
            else
            {
                document.getElementById("txtTotalTime").value= extraTime;
            }
        }
        else if(extraTime!="")
        {
             var totalTime=(time*days*rate*0.01)+parseInt(extraTime);
            document.getElementById("txtTotalTime").value= totalTime;
        }
        else
        {
             var totalTime=(time*days*rate*0.01);
            document.getElementById("txtTotalTime").value= totalTime;
        }
    }
   
    function days()
    {
        var year= document.getElementById("txtYearMonth").value;
        var month=year.substr(5,2);
        year=year.substr(0,4);
        var days=new  Date(year,month,0).getDate();
        document.getElementById("txtWorkDays").value=days;
    }
   
   
    </script>

<asp:TextBox ID="txtYearMonth" runat="server" onpropertychange="days();"></asp:TextBox>
        <br />
        <asp:TextBox ID="txtWorkTime" runat="server" OnTextChanged="txtWorkTime_TextChanged" onpropertychange="txtChange();">